Wow that looks really good! It's a really good concept too - it reminds me of those 20 in 1 Atari joysticks that plug up to your TV, except this one is a *bit* more substantial than those

This sort of project would be great for giving a new life to an old PC without having to dedicate a fraction of your life and wallet to it, I've got a few computers laying around the house not being used and after seeing your project I'm tempted to turn them into a 5000 in 1 machine.
The fact that it is very portable is great, too. Does the videocard have a video-out or just the monitor connection? If it were to have a video out It would be a lot of fun to take around to a friends house and play arcade classics on their TV rather than having to steal their monitor.
Have you considered making it more than just a Mame machine? you could have a nice portable jukebox assuming the mobo has s/pdif out, or even a dedicated server for gaming if you're into that sort of thing!
With or without the extra stuff I mentioned, this is a really nice little arcade console and I think it would be great to have one of these to play against friends without having to lug a huge arcade machine around.